This invention relates to a precision automatic assembly apparatus, and to an assembly method using the apparatus, for performing a basic task such as pin erection automatically and in an accurate manner.
The insertion of pins into holes formed in parts can be performed without difficulty providing that the insertion operation is carried out under a condition where the pins are positioned to be vertical with respect to the corresponding holes and in such a manner that the pin centers coincide with the hole centers. However, automating an actual operation of this kind is accomplished by using a robot or automatic assembly machine in which highly precise positioning and inclination control are difficult to achieve. The smaller the clearance between the pin and hole, the greater the difficulty becomes. Briefly stated, the problems are as follows: (1) It is difficult to bring the centers of the pin and hole into alignment. (2) A mechanism for grasping the pins is required and involves difficulty in eliminating pin inclination error. (3) It is required that each pin be lowered into the corresponding hole along the central axis of the hole, but it is difficult to lower the pin exactly along the vertical. These problems are a source of error in the pin erecting operation.
Unless the foregoing problems are solved, the pins cannot be inserted into the holes and can cause scoring of the holes along their length.
To this end, a hand has been developed for the purpose of bringing a pin and hole into relative registration and correcting the error in pin attitude (inclination).
One example of such a technique is to provide the hand with flexibility so that a pin can be registered with a hole automatically (passively). A typical example of such an arrangement is an RCC (remote center compliance) system in which mechanisms and springs are specially designed to readily move in a direction which will automatically reduce a positional offset between a pin and hole and an error in inclination during the process of inserting the pin.
The construction of an RCC hand of this kind will now be described with reference to FIGS. 1 and 2.
As shown in FIGS. 1 and 2, the RCC hand mechanism includes a portion 1 joined to a hand, a transverse compliance link 2, a rotational compliance link 3, a compliance center 4, a translational portion 5, a rotational portion 6, a table 7 and a hole 8 provided in the table 7.
This arrangement is suitable for an operation in which a pin is inserted into a chamfered hole with a small clearance and comprises, in combination, the translational portion 5 composed of links forming a parallelogram and the rotational portion 6 composed of links forming a trapezoid. FIG. 2 illustrates the links as they would appear if shown in series. The pin to be inserted into the hole is attached to the lower end of the combined links. Accordingly, if a vertically directed force acts upon the pin, the pin will move in the direction of the applied force while its attitude is maintained, owing to the motion of the links forming the parallelogram. When a rotational force is applied, the links forming the trapezoid act about the compliance center 4, whereby the pin undergoes rotational motion. Thus, if the hole 8 is chamfered and the lower end of the pin abuts against the chamfered portion, the pin is acted upon by a transversely directed force and is moved in the direction of the hole center at the same time that it is urged into the hole. If the pin is inserted at an incline, rotational motion is produced about the compliance center 4 so that the pin is moved in a direction that will bring its center line into coincidence with the center line of hole.
Examples of an RCC mechanism of the above-described type are disclosed in e.g. the specifications of U.S. Pat. Nos. 4,098,001, 4,439,926 and 4,477,975 (C1. 33).
In accordance with the aforementioned prior art, the dimensions of the mechanism and the distribution of the spring constants are so decided that the center position of the RCC hand will agree with the tip of the pin. If the length of the pin to be inserted is changed, therefore, the RCC hand will no longer be effective. In other words, it is necessary to use an RCC hand specially designed for the particular activity, so that the hand lacks versatility. In addition, since the springs used have considerable flexibility, vibrations occur when the pin is moved to approach the hole. As a result, the speed of the overall assembly operation declines and the efficiency of the operation suffers.